The present invention relates broadly to thermal management materials for electronic devices. Such materials commonly are used as heat transfer interfaces between, for example, the mating heat transfer surfaces of a heat-generating, electronic component, such as an integrated circuit (IC) chip, and a thermal dissipation member, such as a heat sink, for the conductive cooling of the electronic component. More particularly, the present invention relates to a release liner for tapes and pads of such materials which are adhered to the heat transfer surface of the heat sink prior to the installation of the heat sink in the electronic device. In accordance with the invention, such liner is zone-coated with a release agent to provide a controlled release value or peel strength. When the liner is used as a protective sheet for the exterior surface of a pad which is adhered to the heat sink, such zone coating assists in assuring that the liner remains affixed to the pad so that the heat sink can be handled or shipped, and in allowing prior to installation the clean removal of the liner without lifting the pad from the heat transfer surface to which it has been applied.
Circuit designs for modern electronic devices such as televisions, radios, computers, medical instruments, business machines, communications equipment, and the like have become increasingly complex. For example, integrated circuits have been manufactured for these and other devices which contain the equivalent of hundreds of thousands of transistors. Although the complexity of the designs has increased, the size of the devices has continued to shrink with improvements in the ability to manufacture smaller electronic components and to pack more of these components in an ever smaller area.
As electronic components have become smaller and more densely packed on integrated boards and chips, designers and manufacturers now are faced with. the challenge of how to dissipate the heat which is ohmicly or otherwise generated by these components. Indeed, it is well known that many electronic components, and especially power semiconductor components such as transistors and microprocessors, are more prone to failure or malfunction at high temperatures. Thus, the ability to dissipate heat often is a limiting factor on the performance of the component.
Electronic components within integrated circuit traditionally have been cooled via forced or convective circulation of air within the housing of the device. In this regard, cooling fins have been provided as an integral part of the component package or as separately attached thereto for increasing the surface area of the package exposed to convectively-developed air currents. Electric fans additionally have been employed to increase the volume of air which is circulated within the housing. For high power circuits and the smaller but more densely packed circuits typical of current electronic designs, however, simple air circulation often has been found to be insufficient to adequately cool the circuit components.
Heat dissipation beyond that which is attainable by simple air circulation may be effected by the direct mounting of the electronic component to a thermal dissipation member such as a xe2x80x9ccold platexe2x80x9d or other heat sink or spreader. The dissipation member may be a dedicated, thermally-conductive ceramic or metal plate or finned structure, or simply the chassis or circuit board of the device. However, beyond the normal temperature gradients between the electronic component and the dissipation member, an appreciable temperature gradient is developed as a thermal interfacial impedance or contact resistance at the interface between the bodies.
That is, and as is described in U.S. Pat. No. 4,869,954, the faying thermal interface surfaces of the component and heat sink typically are irregular, either on a gross or a microscopic scale. When the interfaces surfaces are mated, pockets or void spaces are developed therebetween in which air may become entrapped. These pockets reduce the overall surface area contact within the interface which, in turn, reduces the heat transfer area and the overall efficiency of the heat transfer through the interface. Moreover, as it is well known that air is a relatively poor thermal conductor, the presence of air pockets within the interface reduces the rate of thermal transfer through the interface.
To improve the heat transfer efficiency through the interface, a layer of a thermally-conductive, electrically-insulating material typically is interposed between the heat sink and electronic component to fill in any surface irregularities and eliminate air pockets. Initially employed for this purpose were materials such as silicone grease or wax filled with a thermally-conductive filler such as aluminum oxide. Such materials usually are semi-liquid or solid at normal room temperature, but may liquefy or soften at elevated temperatures to flow and better conform to the irregularities of the interface surfaces.
The greases and waxes of the aforementioned types heretofore known in the art, however, generally are not self-supporting or otherwise form stable at room temperature and are considered to be messy to apply to the interface surface of the heat sink or electronic component. To provide these materials in the form of a film which often is preferred for ease of handling, a substrate, web, or other carrier must be provided which introduces another interface layer in or between which additional air pockets may be formed. Moreover, use of such materials typically involves hand application or lay-up by the electronics assembler which increases manufacturing costs.
Alternatively, another approach is to substitute a cured, sheet-like material or pad for the silicone grease or wax material. Such materials may be compounded as containing one or more thermally-conductive particulate fillers dispersed within a polymeric binder, and may be provided in the form of cured sheets, tapes, pads, or films. Typical binder materials include silicones, urethanes, thermoplastic rubbers, and other elastomers, with typical fillers including aluminum oxide, magnesium oxide, zinc oxide, boron nitride, and aluminum nitride.
Exemplary of the aforesaid interface materials is an alumina or boron nitride-filled silicone or urethane elastomer which is marketed under the name CHO-THERM(copyright) by the Chomerics Division of Parker-Hannifin Corp., 16 Flagstone Drive, Hudson, N.H. 03051. Additionally, U.S. Pat. No. 4,869,954 discloses a cured, form-stable, sheet-like, thermally-conductive material for transferring thermal energy. The material is formed of a urethane binder, a curing agent, and one or more thermally conductive fillers. The fillers, which may include aluminum oxide, aluminum nitride, boron nitride, magnesium oxide, or zinc oxide, range in particle size from about 1-50 microns (0.05-2 mils).
Sheets, pads, and tapes of the above-described types have garnered general acceptance for use as interface materials in the conductive cooling of electronic component assemblies such as the semiconductor chips, i.e., dies, described in U.S. Pat. No. 5,359,768. In certain applications, however, heavy fastening elements such as springs, clamps, and the like are required to apply enough force to conform these materials to the interface surfaces in order to attain enough surface for efficient thermal transfer. Indeed, for some applications, materials such as greases and waxes which liquefy, melt, or soften at elevated temperature continue to be preferred as better conforming to the interface surfaces under relatively low clamping pressures.
Recently, phase change materials have been introduced which are self-supporting and form-stable at room temperature for ease of handling, but which liquefy or otherwise soften at temperatures within the operating temperature range of the electronic component to form a viscous, thixotropic phase which better conforms to the interface surfaces. These phase change materials, which may be supplied as free-standing films, or as heated screen printed onto a substrate surface, advantageously function much like greases and waxes in conformably flowing within the operating temperature of the component under relatively low clamping pressures of about 5 psi (35 kPa). Such materials are further described in co-pending U.S. application Ser. No. 08/801,047, filed Feb. 14, 1997, and in counterpart International Publication No. WO 97/41599, and are marketed commercially under the names THERMFLOW(trademark) T310, T443, T705, T710, and T725 by the Chomerics Division of Parker-Hannifin Corp., 16 Flagstone Drive, Hudson, N.H. 03051. Other phase change materials are marketed commercially by the Bergquist Company (Minneapolis, Minn.) under the tradename xe2x80x9cHI-FLOW(trademark),xe2x80x9d and by Orcus, Inc. (Stilwell, Kans.) under the tradename xe2x80x9cTHERMAPHASE.xe2x80x9d
For typical commercial application, the thermal interface material may be supplied in the form of a tape which includes an inner and outer release liner and an interlayer of thermal compound. As most thermal compounds are not inherently tacky, one side of the compound layer may be coated with a thin layer of a pressure-sensitive adhesive (PSA) for the application of the compound to the heat transfer surface of a heat sink. In order to facilitate automated dispensing and application, the outer release liner and compound interlayer of the tape may be die cut to form a series of individual, presized pads. Each pad thus may be removed from the inner release liner and bonded to the heat sink using the adhesive layer in a conventional xe2x80x9cpeel and stickxe2x80x9d application which typically is performed by the heat sink manufacturer.
With the pad being adhered to the heat transfer surface of the thermal dissipation member such as a heat sink or spreader, and with the outer liner in place to form a protective cover the outer surface of the compound layer, the dissipation member and pad may be provided as an integrated assembly. Prior to installation of the assembly, the outer release liner from the compound layer, and the pad positioned on the electronic component. A clamp may be used to secure the assembly in place.
It will be appreciated that the protective outer release liner must exhibit a peel strength or xe2x80x9crelease valuexe2x80x9d relative to the compound layer which is high or xe2x80x9ctightxe2x80x9d enough to prevent the separation of the liner during shipping and handling of the assembly, but which is low or xe2x80x9cloosexe2x80x9d enough to allow for the clean removal of the liner on aging without causing a cohesive failure of the compound layer or an adhesive failure of the adhesive layer. That is, if the liner is too loose, there is the potential that it may separate from the pad during shipping or handling and thereby subject the compound layer to the risk of contamination or other damage. However, if the liner is too tight, it may be difficult to remove without splitting the compound or lifting the pad from the heat transfer surface. In each of these situations, the assembly may have to be discarded.
Release liners or sheets conventionally have involved a film base of polyester, polyethylene terephthalate, plasticized polyvinyl chloride, cellulose, metal foil, polypropylene, polystyrene, or polyethylene which is coated with a wax, silicone, or fluoropolymer to reduce the surface energy of the base and to allow the liner to be removed without appreciable lifting of the adhesive layer. Alternatively, bare films of fluoropolymers and silicones have been employed.
With respect to the thermal interface materials herein involved, however, it has been observed in some circumstances that the coated release liners or uncoated fluoropolymer or silicone films heretofore known in the art have proved too loose to assure no premature separation from the material pad. Likewise, with respect to other uncoated release liners heretofore known in the art, it has been observed in some circumstances that such liners have proved too tight resulting in adhesive or cohesive failure of the pad. Accordingly, it will be appreciated that improvements in release liners for phase change and other interface materials would be well-received by industry. A preferred liner would resist premature separation from the interface material, but would allow for a clean, controlled release prior to the use.
The present invention is directed to a thermal management materials, and particularly to release liners for tapes and pads of such materials which are adhered to the heat transfer surface of a heat sink or spreader prior to its installation in an electronic device. In accordance with the precepts of the invention, the liner is provided as having one or more first zones exhibiting a given first release value or peel strength relative to the interface material, and one or more second zones bordered by the first zones which exhibit a second release value or peel strength which is lower than that of the first zones. The ratio of the surface area of the first zone or zones to that of the second zone or zones, which typically will be between about 10-90% for most applications, may be selected to a effect an overall balanced release value for the liner. When employed, for example, as a protective liner for pads of thermal interface materials adhered to the heat transfer surface of a heat sink, heat spreader, or the like, such liner substantially reduces the occurrence of premature separation during shipping, handling, or storage of the part. However, prior to the installation of the part, the liner may be controllably removed from the interface pad without evident adhesive or cohesive failure.
In an illustrative embodiment, the release liner of the present invention includes a film base of which may be a polyester, polyolefin, fluoropolymer, or other polymer. The second zones, which may be discrete as defined in a striped, diamond, circular, polygonal, elliptical, or other rectilinear or arcuate shape, or as defined in a continuous pattern by a pattern of discrete first zones, are coated with a silicone or other release agent. The first zones, which may be left uncoated, typically will exhibit a release value relative to the thermal interface pad of greater than about 800 g/in, in contrast to the second zones which typically will exhibit a release value of between about 10-500 g/in. Alternatively, the first zones also may be coated with a different coating having a higher release value than the coating of the second zones, or with the same coating which is applied in a thinner layer or at a lower solids content. The coating or coatings may be applied in a conventional manner, for example, by a direct process such as spraying, dipping, casting, or knife, roller, gravure, wire rod, or drum coating, an indirect transfer process, or by coating the entirety of the surface and then removing the coating from the first zones by etching, coronal discharge, or other means.
The invention, accordingly, comprises the apparatus and method possessing the construction, combination of elements, and arrangement of parts and steps which are exemplified in the detailed disclosure to follow. Advantages of the invention include a zone coated release liner which is particularly adapted for use as a protective liner on phase change and other thermal interface materials in allowing for a controlled release from the surface thereof. Additional advantages include a zone coated release liner having a release value which may be economically tailored to suit the particular application involved. These and other advantages will be readily apparent to those skilled in the art based upon the disclosure contained herein.